In web and coating manufacturing operations, expensive bulk optic interferometric apparatuses are used for accurate, on-line measurements of web and coating layer thickness. An apparatus, such as shown in U.S. Pat. No. 5,633,712, by Venkatesh, et al., U.S. Pat. No. 5,659,392, by Marcus, et al. which issued Aug. 19, 1997 and the associated method taught in U.S. Pat. No. 5,596,409 by Marcus, et al. which issued Jan. 21, 1997 have a high degree of lateral resolution, are light weight, compact, easy to set up, and are robust in high and low temperature environments, in the presence of solvents, high air flow, and various levels of relative humidity. Such apparatuses are self-calibrating or able to remain in calibration for extended periods of time so that the apparatus can be installed on a production machine without the need for re-calibration. Unfortunately, the expensive bulk optics and mechanics required in such devices reduce their usefulness except in high value production applications. In addition, the mechanical nature of the scanning optics of such devices reduce the possible scan rate and service life.
Therefore, there has been a need for an economical, long lifetime measuring device, which can produce accurate measurements at high scan rates that require little periodic maintenance and can be packaged with minimal size and mass.